Blender 2.8: Very hard to see selected vertices & edges in edit mode
Since the edge edit mode in your screenshot is displayed with default line width, you may want to check the overlay's option in the 3D View panel header bar.

After entering the edit mode, Mesh Edit Mode options will appear in there. Without the center checkbox been checked, the line display in edge mode will be slightly bigger then normal to make difference from face mode.



And for vertex mode,
you can change directly in Setting &gt;&gt; Themes &gt;&gt; 3D View &gt;&gt; Certex Size, but the line width won't change either.

An alternative way is changing Setting &gt;&gt; Interface &gt;&gt; Line Width to thic, making literally everything bigger than usual. Which might not be the best solution.